Electrophotographic

Technology

Electrophotographic printing was invented in the 1930’s by Chester Carlton. By the late 1950’s, electrophotography (also referred to as xerography after the Xerox Company) was the most popular form of office copying. Original documents could be placed face-down on a glass platen, and in moments an adequate facsimile was produced. In the mid 1970’s the process was adapted for use as a hardcopy output method for computers. Instead of using light reflected from an original document as in the old photo-copiers, a laser (sometimes an LED) was used to translate computer data into light pulses that would expose a light sensitive, photo-conducting drum or belt (see below). By the 1980’s laser printing had been downscaled enough to be manufactured within a desktop-sized printer. The following are an illustration and a description of the six basic stages to this form of printing:

Charging of the Photo-Conductor - A photo-conducting drum or a belt is charged by a corona (electrical discharge).

Exposure - When light exposes the charged drum or belt, the charge becomes neutralized. In a laser printer, the laser is turned on and off by the digital data from the computer. The laser is “on” for the white areas of the document or image, and it neutralizes the charge on those areas of the drum. The black areas of the document or image are not exposed by the laser, so those areas of the drum remain charged.

Development – In this step, the toner is applied to the surface of the exposed drum. The toner carries an opposite charge from the drum, so toner only adheres to the still charged, unexposed areas.

Transfer – The toner is now transferred from the drum to the paper. A second corona is positioned behind the paper, and it creates a charge greater than that of the photo-conductor. This pulls the toner from the drum to the paper.

Fusing – At this point the toner is only resting on the surface of the paper and can easily be smeared. To fix the image, a variety of techniques may be employed. For dry-toner systems, the fixing process is a combination of heat and pressure. For liquid-toner systems, evaporation of the solvent dries the image.

Cleaning – in order for the photo-conducting drum or belt to be reused for the next print, it must be cleaned of both residual toner and charge. The toner is scraped off mechanically. The drum is then cleaned of charge by a light that neutralizes any remaining charge. The system is now ready to begin again.

For an electrophotographic printer to create color images, four separate “impressions” (one each for cyan, magenta, yellow, and black toners) must be made.

Image Structure

Because electrophotographic printers can only produce a single, dark tone for each of its colors (whether monotone black or four-color CMYK), they must create shades of color by utilizing the white of the paper. Dark colors are created by placing dots of toner close together while light colors are created by placing dots further apart. This process is similar to the screening technique used in offset printing known as halftoning.

Dry toner image pattern

Liquid toner image pattern

The individual image dots are made up of multiple toner particles. Dry toner prints present a typical dusty appearance produced by toner particles that fall outside the intended dot area. Liquid toner, instead, may present fine toner particles, but always within the dot area.

Colorants

The coloring agents used in electrophotographic systems are called toners. They come in two forms – dry and liquid. Dry toners are used in all desktop and office printer/copier systems. Both dry and liquid toner systems are used in electrophotographic digital presses.

Dry toners consist of pigments or dyes embedded inside polymer beads. The fusing process melts the polymer beads to the surface of the paper. In addition to the colorants within the polymer beads are “charge agents” that allow the toner to be charged opposite to that of the photo conducting drum. Without this charge the toner would not stick to the drum (or belt) or be transferable to the paper. Toners also contain small “lubricating” particles between them to keep them from sticking together. For black-and-white printers the colorant is carbon black which is typically very stable. Therefore, black-and-white electrophotographic prints made on high-quality alkaline papers should be very long-lasting.

Papers

The papers used for electrophotographic can be either uncoated or coated. The majority of desktop or office printer paper is uncoated and is often referred to as office paper or copier paper. A major disadvantage of plain paper for photos is that it is thin and easily bent or torn while handling.

Coated papers are similar to plain papers with the exception that a heavy mineral coating has been applied to the surface during manufacture to increase the papers’ density, opacity, and smoothness. This is often the type of paper we find in glossy magazines. It’s also used in brochures, booklets, and posters. Coated papers are sometimes used for desktop or office electrophotographic printing, and it is the preferred paper for digital presses. The images below show cross-sections of prints on both types of paper. The fused toner particles are clearly visible on top of the paper.